It has been recognized that the hazard potential of radioactive waste can be reduced by separating the long-lived uranium and transuranium elements, hereinafter referred to as actinides, and their subsequent transmutation into short-lived or even stable nuclides. Specific attention has been directed in this regard to so-called high-level liquid wastes (HLLW) which contain, in addition to the actinides, sufficient quantities of lanthanides, i.e. elements ranging in atomic number from 58 to 71, which can effect efficiency of transmutation because of the high neutron absorption cross sections of lanthanides.
Chemical separation of trivalent actinides (An) from lanthanides (Ln) is a difficult separation because of the great chemical and physical similarity of the elements of the two groups and because of the similar ionic radii of the metals and their oxidation characteristics.
Hence efforts have been made to accomplish this separation by extraction techniques. For example, a commercial product known as Cyanex 301 has been identified as capable of yielding a very high Am (III)/Eu(III) separation factor. The key ingredient of the Cyanex 301 for extraction purposes, i.e. the key component of the extractant, is bis(2,2,4 trimethylpentyl)-dithiophosphinic acid.
The extraction practice in earlier techniques of this type at a pH above about 3 which has not been convenient to highly acidic wastes for which such extractions are of the greatest importance.
By and large, therefore, earlier systems for the selective extraction of trivalent lanthanides from acidic aqueous solutions including high-level liquid wastes have not been fully satisfactory. Mention may also be made of the hydrometallurgical technique for separating lower atomic number metals described in U.S. Pat. No. 4,721,605 and using dialkyldithrophosphinic acids.